1. Field of the Invention
The present invention relates to a capacitive occupant sensing apparatus.
2. Description of Related Art
Various types of capacitive occupant sensing apparatuses have been proposed. For example, Japanese Unexamined Patent Publication No. 2008-111809A (corresponding to US2008/0100425A1) teaches a capacitive occupant sensing apparatus shown in FIGS. 9A and 9B, in which a sensing electrode 91 is placed in an inside of a seat 90 of a vehicle, and a guard electrode 92 is placed on a lower side of the sensing electrode 91. A sinusoidal signal (oscillation signal) is applied to the sensing electrode 91 to generate an electric field therearound, and a value of an electric current of the sinusoidal signal, which changes under influence of a conductive body and/or a dielectric body (e.g., a human body) located around the sensing electrode 91, is sensed. Then, according to this sensed result, a corresponding capacitance is computed, and it is determined whether an occupant on the seat 90 is an adult 93 or a child 94 present on a child-restraint system (CRS) based on this computed capacitance.
The electric field, which is generated from the electrode 91, is directly coupled not only with the sensing subject (adult or child) 93, 94 but also with a conductive body 97 (vehicle body), as indicated by circles 95, 96 in FIGS. 9A, 9B. Thereby, the sensed capacitance is increased by the amount, which corresponds to the direct coupling to the conductive body 97.
FIG. 10 shows exemplary data indicating details of capacitances C1, C2 in each sensed capacitance for different cases (different occupants on the seat). In FIG. 10, the capacitance C1 is the capacitance through the human body, and the capacitance C2 is the capacitance increased by the direct coupling.
As shown in FIGS. 9A and 9B, a surface area of the electrode 91, which is covered with the child on the CRS, is smaller in comparison to the adult on the seat. Therefore, in a case where a distance between the electrode 91 and the body of the adult on the seat becomes large due to presence of a cushion between the adult and the seat and/or wearing of thick cloth(es), the capacitance C1 becomes smaller. As a result, in such a case, it is difficult to determine whether the occupant on the seat is the adult or the child on the CRS, as shown in FIG. 10.